Extractive crystallization processes



Patented June 14, 1955 EXTRACTIVE CRYSTALLIZATION PROCESSES BY MEANS OF UREA Alfred Champagnat, Saint-Cloud, France, assignor to The British Petroleum Company Limited No Drawing. Application May 8, 1952, Serial No. 286,829

Claims priority, application France May 17, 1951 14 Claims. (Cl. 260-965) This invention relates to an improved process for the refining of mineral oils. More particularly, the invention relates to a process for the extractive crystallization of petroleum oils, by means of urea.

It is well-known that urea forms crystalline solid adducts with straight chain and slightly branched chain hydrocarbons but does not form solid compounds with naphthenes, highly branched compounds or aromatics. By the use of urea a method has been developed for refining petroleum fractions by resolution of the fractions into chemical types by forming the above solid compounds, removing the remaining liquid phase (known as the urea adduction rafiinate), decomposing the solid compounds and recovering from the product the liberated hydrocarbons (known as the urea adduction extract).

It is also known that the presence of methanol or other lower alcohols in admixture with urea and petroleum fractions undergoing treatment accelerates the rate of formation of the solid derivatives. It has also been stated in the prior art that mixtures of methanol and water may be employed as the activating agent.

In general the methods based on the formation of the adducts by the use of urea in solution have the disadvantage that the urea solutions are apt to form, on cooling, long needles of urea which interlace and form a very large apparent volume. In the industrial application of the process this may lead to the serious disadvantage of the complete blockage of pipes etc.

It is an object of the present invention to provide an improved process for the extractive crystallization of mineral oils, particularly petroleum oils and, more particularly of petroleum oil distillate fractions, by the method of urea adduction wherein the disadvantage of urea crystallization is reduced or eliminated.

It has now been found that in the presence of biuret the rate of crystallization of urea from solution is reduced and that the crystals which are formed are smaller than is the case when biuret is absent.

According to the present invention, the above object is accomplished by a process which comprises treating a mineral oil with a solution of urea in water or in a solvent comprising water, in the presence of biuret, under conditions such that a solid urea adduct is formed with components of the mineral oil and thereafter separating the solid urea adduct from the liquid phase. Preferably the amount of biuret employed constitutes 0.25%, based on the weight of urea present.

The simplest method of providing biuret for use in the process of the present invention consists in promoting in the whole of the urea employed in the process, or preferably only in part of it, the formation of a sutficient proportion of biuret. This may be effected by heating the urea to temperatures between 140 and 170 C. It is known, that at these temperatures two molecules of urea give, with elimination of ammonia, one molecule of biuret, according to the equation:

If the heating of urea is prolonged, or if urea is heated to higher temperatures, then, in addition to biuret, other decomposition products of urea are formed, the principle of which is cyanuric acid formed according to the equation:

These decomposition products of urea used singly or in combination inhibit the crystallization of urea from solution in the same manner as pure biuret.

Preferably the urea solution employed is a solution of urea in a mixture of water and a water-soluble monohydric alcohol or ketone.

More particularly it is preferred that the urea solution employed is a solution of urea in the ternary solvent described in copending U. S. patent application Serial No. 278,627, filed March 26, 1952. It is also preferred that the process of urea adduction be carried out in the manner described in said application. Thus by the process therein described the urea adduct is formed as a suspension in a liquid phase comprising a solution of urea in the ternary solvent, the rafiinate being present as an emulsion in the urea solution.

A particularly preferred ternary solvent for use in the process of the present invention comprises methanol, water and ethylene glycol. Suitable proportions are:

60 parts by weight of methanol 15 parts by weight of water 25 parts by weight of ethylene glycol It has been established that starting with a saturated solution of urea in a ternary solvent as described and rapidly cooling this solution by about 15 C. with vigorous agitation, at supersaturated solution is formed from which is obtained urea crystals which could be termed nascent and which are of very small dimensions. According to the present invention the saturated urea solution is prepared in the presence of biuret. The crystals which form in the presence of the biuret are of even smaller size and react particularly easily with normal aliphatic hydrocarbons.

Thus according to a particularly preferred manner of operating according to the process of the invention, the urea adducts are formed while the urea solution comprising biuret is in a supersaturated state. In practice this may be achieved by cooling a saturated solution of urea containing biuret so as to obtain nascent urea crystals, then forming an emulsion from the supersaturated solution of urea and the hydrocarbon fraction. It is also possible to bring into contact the hydrocarbon fraction and the saturated urea solution and then to cause the nascent crystallization of urea by cooling. This may be efiiected in practice by delivering the urea solution to the contacting zone at a temperature above that of the mineral oil, While containing an excess of urea over that necessary to saturate the solution at the temperature of adduct formation. This method allows the urea crystals to react, from the time of their formation, with the hydrocarbon fraction and to attain a better heat economy since the cooling of the urea solution is efiected by the fresh charge of cool hydrocarbons which thus is warmed up.

The invention is illustrated but in no way limited with reference to the following examples.

EXAMPLE 1 A solution of urea with a content of biuret was prepared using the ternary solvent glycol-methanol-water.

The solution had the following composition (in per cent by weight):

Urea with 1% of biuret 40.5 Glycol 15.2 Methanol (96%) 34.3 Water 10.0

The urea containing 1 per cent of biuret was obtained by mixing 3 parts of commercial urea and 1 part of urea containing 4 per cent of biuret.

Urea containing 4 per cent of biuret had been prepared by subjecting pure commercial urea to prolonged heating at 145 C.

The saturation temperature of the above mentioned solution of urea is 38 C.

100 parts of gasoline obtained by direct distillation of crude Middle East stock and with the boiling range of 90 to 140 C. was brought into contact with 300 parts of the above solution of urea containing biuret at a temperature of 38 C. The mixture was cooled to a temperature of 13 C. while vigorously agitating the agitation being continued after reaching that temperature. The emulsion formed was very highly dispersed and was also found to be more mobile than one prepared without biuret. After agitating for one hour the reaction was completed.

In order to break the emulsion parts of 90% by weight aqueous methanol was added. Due to the presence of biuret, which promotes the formation of urea crystals of very minute dimensions, the emulsion broke easily. The upper raffinate was separated and the remaining layer warmed to decompose the urea adduct. The extract phase came to the surface and was separated by decantation.

The results obtained by this treatment are as follows:

Octane Number The yield of the operation was 85% by weight of ratfinate.

EXAMPLE 2 Treatment of an aviation gas-turbine fuel in the presence of biuret The solution of urea containing biuret was as described in Example 1.

The material treated was a fraction from the direct distillation of a crude Middle East oil with a boiling range of 140 to 320 C. and a temperature of incipient crystallization (cold test) of 20 C. (I. P. Standard Methods of Testing, 1951, page 102).

100 parts of this fraction of hydrocarbons was brought into contact at a temperature of 38 C. with 600 parts of a solution of urea of the composition given in Example 1. The mixture was cooled, while vigorously agitating, to a temperature of 20 C. and agitation continued at that temperature. The formation of the adduct took place simultaneously with the formation of a finely dispersed and mobile emulsion. The progress of the reaction was followed by estimating the specific gravity of the raffinate in samples taken from time to time and subjected to centrifuging. After one hour, when the specific gravity of the raffinate had reached its maximum, the emulsion was broken by adding 40 parts of 90% aqueous methanol and the upper raflinate layer separated by decantation. The remaining layer, containing the adduct in the form of a suspension, was warmed to 40 C. to decompose the complex. The extract phase came to the surface and was separated by decantation.

The following results were obtained in this treatment:

Density at 20 C. (gms./ Cold Test litre) Crude motor fuel finate 20 C. 820.6 lower than 60 C.

The yield was 78% by weight of a raffinate suitable for use as an aviation gas-turbine fuel, the cold test of which conforms to the requirements laid down for trans-oceanic flights.

Although the invention has been described with reference to certain specific embodiments thereof, it is to be distinctly understood that various modifications and adaptations of the arrangements herein disclosed may be made as may readily occur to persons skilled in the art without constituting a departure from the spirit and scope of the invention as defined in the objects and in the appended claims.

What I claim is:

1. A process for the extractive crystallization of mineral oils which comprises treating a mineral oil with a solution of urea in a solvent comprising water, in the presence of biuret in the proportion of 0.2 to 5% by weight relative to the urea content, whereby a solid urea adduct is formed with some components of said mineral oil, separating said solid urea adduct from the remaining components of said mineral oil, decomposing said urea adduct and recovering the liberated components of said mineral oil.

2. A process as specified in claim 1 in which said solution of urea is a solution of urea in a solvent comprising water and a water-soluble mono-hydric alcohol.

3. A process as specified in claim 1 in which said solution of urea is a solution of urea in a solvent comprising water and a water-soluble ketone.

4. A process as specified in claim 1 in which said solvent comprises water, a water-soluble oxygen-containing organic compound selected from the group comprising iLIJlOl'lGS and mono-hydric alcohols and together therewith an alcoholic organic compound selected from the group comprising polyhydric alcohols and moncraminoalcohols.

5. A process for the extractive crystallization of mineral oils which comprises treating a mineral oil with a supersaturated solution of urea in a solvent comprising water, in the presence of biuret in the proportion of 0.2- 5% by weight relative to the urea content, whereby a solid urea adduct is formed with some components of said mineral oil, separating said solid urea adduct from the remaining components of said mineral oil, decomposing said urea aduct and recovering the liberated components of said mineral oil.

6. A process as specified in claim 5 in which said biuret has been produced by heating urea to a temperature in the range l40-170 C.

7. A process for the extractive crystallization of distillation fractions of petroleum which comprises treating a distillation fraction of petroleum with a supersaturated solution of urea in a solvent comprising water, in the presence of biuret in the proportion of 0.2-5% by weight relative to the urea content, whereby a solid urea adduct is formed with some components of said distillation fraction, separating said solid urea adduct from the remaining components of said distillation fraction, decomposing said urea adduct and recovering the liberated components of said distillation fraction.

8. A process as specified in claim 7 in which said solution of urea is a solution of urea in aqueous methanol.

9. A process asspecified in claim 7 in which said solution of urea is a solution of urea in a solvent comprising water, methanol and ethylene glycol.

10. A process for the extractive crystallization of mineral oils which comprises treating a mineral oil with a solution of urea in a solvent comprising water, in the presence of cyanuric acid in the proportion of 0.2 to 5% by weight relative to the urea content whereby a solid urea adduct is formed With some components of said mineral oil, separating said solid urea adduct from the remaining components of said mineral oil, decomposing said urea adduct and recovering the liberated compo nents of said mineral oil.

11. A process as specified in claim 10 in which the cyanuric acid has been produced by heating urea to a temperature above 170 C.

12. A process for the extractive crystallization of mineral oils which comprises treating a mineral oil with a solution of urea in a solvent comprising water, in the presence of a compound selected from the group consisting of biuret and cyanuric acid, in the proportion of 0.2 to 5% by Weight relative to the urea content, whereby a solid urea adduct is formed with some components of said mineral oil, separating said solid urea adduct from the remaining components of said mineral oil, decomposing said urea adduct and recovering the liberated components of said mineral oil.

13. A process as specified in claim 1 in which said biuret is in the proportion of about 1% by weight relative to the urea content.

14. A process for the extractive crystallization of mineral oils which comprises treating a mineral oil with a supersaturated solution of urea in a solvent essentially comprising, in admixture, water, a water-soluble oxygencontaining organic compound selected from the group consisting of ketones, mono-hydric alcohols and mixtures of ketones and mono-hydric alcohols and an alcoholic organic compound selected from the group consisting of polyhydric alcohols, mono-amino-alcohols and mixtures of poly-hydric alcohols and mono-amino-alcohols, in the presence of biuret in the proportion of 0.25% by weight relative to the urea content; forming an em sion of the oil-in-Water type; maintaining said emulsion under conditions such that a solid urea adduct is formed with components of said mineral oil; separating said solid urea adduct from the remaining components of said mineral oil; decomposing said urea adduct and recovering the liberated components of said mineral oil.

References (Iited in the file of this patent Das-Gupta; Chemical Abstracts, vol. 28, page 4706 (1934).

Marecek: Chemical Abstracts, page 2946 (1949). 

1. A PROCESS FOR THE EXTRACTIVE CRYSTALLIZATION OF MINERAL OILS WHICH COMPRISES TREATING A MINERAL OIL WITH A SOLUTION OF UREA IN A SOLVENT COMPRISING WATER, IN THE PRESENCE OF BIURET IN THE PROPORTION OF 0.2 TO 5% BY WEIGHT RELATIVE TO THE UREA CONTENT, WHEREBY A SOLID UREA ADDUCT IS FORMED WITH SOME COMPONENTS OF SAID MINERAL OIL, SEPARATING SAID SOLID UREA ADDUCT FROM THE REMAINING COMPONENTS OF SAID MINERAL OIL, DECOMPOSING SAID UREA ADDUCT AND RECOVERING THE LIBERATED COMPONENTS OF SAID MINERAL OIL. 